1. Field of the Invention
The present invention relates to a circuit arrangement for conference circuits in digital communications systems, particularly in PCM telephone systems in which digitized voice signals of the conference participants are added in the conference circuit and are transmittable to all conference participants minus one's own voice signal generated by the respective conference circuit participants themselves.
2. Description of the Prior Art
In conference circuits defined in such a manner, therefore, the voice signals of the three through n conference participants are added and are transmitted to all conference participants minus the voice signal emitted by the individual conference participants. Depending on the type of length of the transmission link between the individual conference participants, however, particularly given interconnection of mixed analog and digital transmission links in conjunction with non-adapted lines, disturbing echo signals occur which greatly deteriorate the comprehension of the transmitted useful signals (voice signals). Since the echo signals of the conference participants add in the same manner as their useful signals and the sum signal, the useful signals and the echo signals of all participants, is transmitted to every individual conference participant, the echo signals can assume the order of magnitude of the useful signals relatively quickly and, therefore, lead to total incomprehensibility of the voice signals for the individual conference participants.
Numerous methods for echo suppression are known for the elimination of these disturbing influences due to the echo signals and, therefore, for example, the use of various echo barriers and, to an increasing degree, the automatic compensation of echo signals by so-called echo compensators. All of the methods provide that the echo suppression occurs immediately at each individual conference subscriber location and, consequently, considerable preperformance must be undertaken for connections having many conference participants.